1. Field of the Invention
The present invention relates to a starter for starting an engine and, more particularly, to a starter having an improved brush holder.
2. Description of Related Art
In recent years, the engine compartment of a vehicle has become dense with auxiliary devices to allow the engine to have high performance and high function. In such a situation, there is a demand for the development of a compact starter to load the vehicle with a lot of auxiliary devices.
To comply with such a demand, it is proposed in Laid-Open Japanese Patent Publication No. 5-332229 that the slidable contact surface of a disk or surface commutator of an armature is disposed substantially perpendicularly to the rotation shaft of the armature. This construction allows the starter to be compact because the entire axial length of the armature is shorter than the conventional one having a cylindrical commutator. Further, in the starter, an electromagnetic switch for controlling the supply of electric current to the armature is positioned axially rearward from the armature. This construction allows the starter to have more auxiliary devices than the biaxial type conventional one in which the electromagnetic switch is positioned radially outward from the starting motor.
However, in the above starter, the movement of the armature is allowed to some extent in the axial direction of its rotation shaft so that the thrust of the armature does not become zero. Because, if the thrust becomes zero, mechanical loss increases and output decreases. In this construction, the armature moves axially due to fluctuation of a load applied to the armature in its rotation when the engine is driven. In addition, the vibration of the engine is amplified, because the electromagnetic switch is positioned rearward from the armature in its axial direction. Consequently, the brush is incapable of stably contacting the surface of the commutator, which deteriorates the commutation and reduces the longevity of the brush. In consideration of the abrasion of the brush, it is necessary to provide the starting motor with a long brush.
Because the electromagnetic switch is positioned rearward from the armature in its axial direction, the brush is proximate to the electromagnetic switch. Therefore, subjected to the heat generated by the electromagnetic switch, the temperature of the brush rises. Consequently, the longevity of the brush is reduced owing to the increase in the resistance generated between the contact of the surface of the commutator and the surface of the brush caused by a temperature rise in both contact surfaces. In addition, the brush holder made of resin for holding the brush has a low degree of thermal conductivity and surrounds the brush to firmly hold the brush therein. Thus, it is difficult to release the heat generated by the brush. This is also the reason which shortens the longevity or life of the brush.
Further, in the above starter, the slidable contact surface of the commutator of the armature is substantially perpendicular to the rotation shaft of the armature, and the electromagnetic switch is positioned at the side opposite to the armature side with respect to the brush holder holding a plurality of brushes which contact the commutator. In keeping the insulation performance of the electric-conductive portion of an electromagnetic switch, the holder is required to be provided with a partitioning wall to prevent the powder of the brush generated by the slidable contact of the surface thereof with the surface of the commutator from penetrating into the space in which the electromagnetic switch is positioned.